Experimental evidence of thermal fluctuations on the X-ray absorption near-edge structure at the aluminum K-edge

After a review of temperature-dependent experimental x-ray absorption near-edge structure (XANES) and related theoretical developments, we present the Al K-edge XANES spectra of corundum and beryl for temperature ranging from 300K to 930K. These experimental results provide a first evidence of the role of thermal fluctuation in XANES at the Al K-edge especially in the pre-edge region. The study is carried out by polarized XANES measurements of single crystals. For any orientation of the sample with respect to the x-ray beam, the pre-edge peak grows and shifts to lower energy with temperature. In addition temperature induces modifications in the position and intensities of the main XANES features. First-principles DFT calculations are performed for both compounds. They show that the pre-edge peak originates from forbidden 1s to 3s transitions induced by vibrations. Three existing theoretical models are used to take vibrations into account in the absorption cross section calculations: i) an average of the XANES spectra over the thermal displacements of the absorbing atom around its equilibrium position, ii) a method based on the crude Born-Oppenheimer approximation where only the initial state is averaged over thermal displacements, iii) a convolution of the spectra obtained for the atoms at the equilibrium positions with an approximate phonon spectral function. The theoretical spectra so obtained permit to qualitatively understand the origin of the spectral modifications induced by temperature. However the correct treatment of thermal fluctuation in XANES spectroscopy requires more sophisticated theoretical tools

Selecting core-hole localization or delocalization in CS2 by photofragmentation dynamics.

Electronic core levels in molecules are highly localized around one atomic site. However, in single-photon ionization of symmetric molecules, the question of core-hole localization versus delocalization over two equivalent atoms has long been debated as the answer lies at the heart of quantum mechanics. Here, using a joint experimental and theoretical study of core-ionized carbon disulfide (CS2), we demonstrate that it is possible to experimentally select distinct molecular-fragmentation pathways in which the core hole can be considered as either localized on one sulfur atom or delocalized between two indistinguishable sulfur atoms. This feat is accomplished by measuring photoelectron angular distributions within the frame of the molecule, directly probing entanglement or disentanglement of quantum pathways as a function of how the molecule dissociates

Structural study of Mg-bearing sodosilicate glasses by Raman spectroscopy

International audienc

Influence du zirconium sur les mobilités de cation dans les liquides Na2O-CaO-Al2O3-SiO2: diffusion multicomposante et étude XANES

International audienceHigh temperature cation mobilities, based on the multicomponent diffusion method, have been coupled with a structural investigation in the Na2O-CaO-Al2O3-SiO2-ZrO2 (NCASZ) system at 1200°C and 1250°C. From structural investigation using X-ray absorption spectroscopy (XANES), zirconium was determined in six-fold coordinated sites in the reduced glass composition range of our study and no significant structural changes have been evidenced compared to a quaternary Na2O-CaO-Al2O3-SiO2 (NCAS) glass with a similar composition. Additionally, the diffusion experiments revealed that adding zirconium in the quaternary melt has no influence on the dominant eigenvectors that correspond to exchange between calcium and sodium. Despite the expected mobility decrease resulting from a viscosity increase upon the addition of zirconium, the dominant eigenvalue calculated in this study is higher when Zr is introduced in the melt structure. This result strongly suggests that the sodium mobility is enhanced by the presence of Zr, which is explained by a change in the structural role of sodium from network modifier associated to Q3(Si) sites in NCAS melts to charge compensator associated with [ZrO6] 2-sites in NCASZ melts. This modification generates lower bond strengths between sodium and other cations in the melt, thus favoring an enhancement in mobility. Moreover, the diffusion matrix was applied to predict diffusion profiles between zirconium-bearing crystals and melts. We observed that even far from the composition field used for the matrix determination, predictions of zirconium diffusion profiles may be relevant and demonstrate the potential of this approach to evaluate crystal/melt dissolution behavior.Les mobilités de cation à haute température, basées sur la méthode de diffusion multicomposante, ont été couplées à une étude structurale dans le système Na2O-CaO-Al2O3-SiO2-ZrO2 (NCASZ) à 1200°C et 1250°C. À partir d’une étude structurale utilisant la spectroscopie d’absorption des rayons X (XANES), le zirconium a été déterminé dans des sites de ccoordinence 6 dans la gamme de composition réduite de notre étude et aucun changement structurel significatif n’a été démontré par rapport à un verre quaternaire Na2O-CaO-Al2O3-SiO2 (NCAS) avec une composition similaire. En outre, les expériences de diffusion ont révélées que l’ajout de zirconium dans la fonte quaternaire n’a aucune influence sur les vecteurs propres dominants qui correspondent à l’échange entre le calcium et le sodium. Malgré la diminution prévue de la mobilité résultant d’une augmentation de la viscosité lors de l’ajout de zirconium, la valeur propre dominante calculée dans cette étude est plus élevée lorsque Zr est introduit dans la structure du liquide. Ce résultat suggère fortement que la mobilité en sodium est renforcée par la présence de Zr, ce qui s’explique par un changement dans le rôle structurel du sodium depuis un rôle de modificateur de réseau associé aux sites Q3(Si) dans les liquides NCAS vers un rôle de compensateur de charge associé aux sites [ZrO6]2- dans les liquides NCASZ. Cette modification génère des forces de liaison plus faibles entre le sodium et d’autres cations dans le liquide, favorisant ainsi une amélioration de la mobilité. En outre, la matrice de diffusion a été appliquée pour prédire les profils de diffusion entre les cristaux porteurs de zirconium et les liquides. Nous avons observé que même loin du champ de composition utilisé pour la détermination de la matrice de diffusion, les prédictions des profils de diffusion du zirconium peuvent être pertinentes et démontrer le potentiel de cette approche pour évaluer le comportement de dissolution à l'interface cristal/liquide

Mg K-edge XANES spectra in crystals and oxide glasses: Experimental vs. theoretical approaches

International audienc

X-Ray Absorption Near Edge Structure (XANES) study of the speciation of uranium and thorium in Al-rich CaSiO3CaSiO_{3} perovskites

International audienc

LUCIA To Decipher Metal Homeostasis And Toxicity In Plants

Some metals such as iron (Fe) are essential to plants whilst others such as cadmium (Cd) are toxic even at low concentration. The identification of the chemical forms of these metals (speciation) and their localization at the tissue and cell level are crucial to understand theirtrafficking and storage and finally the ability for the plant to cope with their potential toxicity or requirement. LUCIA beamline offers the possibility to access the metal distribution thanks to micro X-ray fluorescence with a micrometer sized lateral resolution and to investigate the metal speciation using micro X-ray Absorption Near Edge Structure. Importantly, the beamline is designed for measurements in cryogenic conditions, which is necessary to limit metal redistribution and speciation change in biologic samples. Here, we present twoexamples of our work performed on Lucia during the last years. The first one deals with Fe homeostasis and the discovery of an important pool of iron in plant cell nucleus (1) as well as the Fe species in plant embryos (2). The second example is dedicated to the study of Cd sequestration in Anthyllis vulneraria used as a pioneer plant in the phytostabilization of mining sites (3).REFERENCES1. H. Roschzttardtz, L. Grillet, M.P. Isaure, G. Conejero, R. Ortega, C. Curie, and S. Mari, J. Biol. Chem. 286, 27863-27866 (2011).2. L. Grillet, L. Ouerdane, P. Flis, M. Thi Thanh Hoang, M. P. Isaure, R. Lobinski, C. Curie, and S. Mari, J. Biol. Chem. 289, 2515-2525(2014).3. S. Huguet, S. Soussou, J. C. Cleyet-Marel, N. Trcera, M. P. Isaure, ‘Rhizostabilization of a mine tailing highly contaminated:preliminary study of Cd localization and speciation in Anthyllis vulneraria in ‘Proceedings of the 16th International Conference onHeavy Metals in the Environment’, edited by N. Pirrone, Roma, Italy, 2013, n°19008

Rhizostabilzation of a mine tailing higly contaminated: Previous study of Cd localization and speciation in Anthyllis vulneraria

The plant Anthyllis vulneraria has been identified in mine tailings and the aim of this study is to determine the mechanisms developed by the plant and its symbiotic association Mesorhizobium metallidurans to tolerate Cd. We particulary intend to determine the distribution and speciation of Cd in plant using a combination of μXRF (X-ray fluorescence) and Cd XANES and μXANES (X-ray Absorption Near Edge Structure)